About the Hard Drive Actual Capacity Calculator
This calculator explains the gap between the capacity printed on a hard drive, SSD or USB stick and the number your computer reports. Manufacturers use decimal units, where 1 TB is exactly 1,000,000,000,000 bytes. Windows divides by 1,024 instead of 1,000 at every step but still labels the result "GB" or "TB", so the same trillion bytes appear as about 931 GB. Nothing is missing — it is the same amount of storage measured with a different ruler.
Enter the advertised size to see the exact number of bytes, the size in binary gibibytes (GiB) and tebibytes (TiB), what Windows will display, what macOS displays, and the apparent shortfall in percent. A reference table covers common drive sizes so you can check a purchase at a glance.
The figures are raw capacity. The usable free space after formatting is slightly lower again because the file system reserves room for its own structures, and a new computer's drive also holds the operating system and a recovery partition.
How to use the hard drive actual capacity calculator
- 1Enter the capacity printed on the drive or in the product listing.
- 2Choose the unit — GB or TB for most drives.
- 3Read what Windows will report and what macOS will report.
- 4Check the shortfall percentage and the common-sizes table to compare drives.
Formula and method
Drive makers use SI decimal prefixes: kilo = 1,000, mega = 1,000², giga = 1,000³, tera = 1,000⁴. Windows (and many older tools) measure in binary multiples of 1,024 — strictly kibibytes, mebibytes, gibibytes and tebibytes (KiB, MiB, GiB, TiB) as defined by the IEC — but label them KB, MB, GB and TB. Dividing the same byte count by 1,024ᵏ instead of 1,000ᵏ gives a smaller number.
The gap compounds with each prefix: about 2.3% at kilo, 4.6% at mega, 6.9% at giga, 9.1% at tera and 11.2% at peta. macOS (since Mac OS X 10.6 Snow Leopard), iOS 11 and later, and GNOME's file manager on Linux use decimal units, so they show the advertised figure; Apple notes that iOS 10 and earlier and OS X Leopard and earlier reported binary values. The calculator picks the largest binary unit that keeps the value at 1 or more, the way Windows Explorer does.
- k
- Prefix power: 3 for giga, 4 for tera, 5 for peta
- size
- Advertised (decimal) capacity
Worked examples
Why a 1 TB drive shows 931 GB
1 TB is 10¹² bytes. Dividing by 1,024³ = 1,073,741,824 gives 931.32, which Windows labels "GB". The 68.68 "missing" GB is purely the unit difference: 1 − (1000/1024)³ ≈ 6.87%.
4 TB external hard drive
4 × 10¹² bytes ÷ 1,024⁴ = 3.638 TiB, displayed by Windows as about 3.64 TB. At the tera level the gap is 1 − (1000/1024)⁴ ≈ 9.05%.
256 GB SSD
256 × 10⁹ = 256,000,000,000 bytes, and dividing by 1,073,741,824 gives 238.42 GiB — the "238 GB" you see in Windows before any files are installed.
Frequently asked questions
Why does my 1 TB hard drive only show 931 GB?+
The manufacturer counts 1 TB as 1,000,000,000,000 bytes, but Windows divides by 1,024 three times (1,073,741,824) and still calls the result GB. 10¹² ÷ 1,073,741,824 = 931.32, so no storage is actually missing.
What is the difference between TB and TiB?+
A terabyte (TB) is 1,000⁴ = 10¹² bytes. A tebibyte (TiB) is 1,024⁴ = 1,099,511,627,776 bytes, about 9.95% larger. Windows shows TiB values but labels them TB.
Does macOS show the full capacity?+
Yes. Since Mac OS X 10.6 Snow Leopard, macOS uses decimal units, so a 1 TB drive appears as 1 TB (before the system and formatting overhead are subtracted). iOS has done the same since iOS 11; Apple notes that iOS 10 and earlier and OS X 10.5 Leopard and earlier used binary units.
How much usable space will I actually get on a new drive?+
Start with the Windows figure from this calculator, then subtract a small amount for the file system and any partitions. A blank 1 TB data drive typically shows about 931 GB total; a laptop system drive also holds the OS, apps and a recovery partition.
Is the capacity on the box misleading?+
It follows the SI definition of the prefixes, which the IEC and NIST endorse for decimal units, and drive makers disclose that 1 GB = 1 billion bytes. The confusion comes from operating systems using binary multiples with decimal names.